Amino Acids
building blocks of protein contain amino (-NH2) & carboxyl(-COOH) functional groups
differ from one another based on the chemical composition of the R group ( side chain )
amino acid - formation of peptide bonds
the amino group of one amino acids can be linked with the carboxyl group of another to form a peptide bond
polypeptide: a chain of amino acids linked by peptide bonds
protein: a large polypeptide
Amino acid metabolism
20 amino acids are necessary for synthesis of proteins
10 of these are essential amino acids ( must come from diet)
* don’t need to memorize them
amino acid pool
pepsin & trypsin digest dietary proteins into amino acids which are absorbed from the intestine into the bloodstream
can be used to synthesize body proteins, hormones, nucleic acids, etc.
proteins provide up to 20% of daily required energy needs
deamination of amino acids produces NH4+which is converted to urea in the liver
ketoacids enter common metabolic pathway with carbs & fats & are converted to energy
ketogenic amino acids
- can be degraded to acetly- CoA to then form ketone bodies
- ex. leucine, lysine
Glucogenic amino acids
- can be converted ti glucose through gluconeogenesis
- 13 amino acids are glucogenic
Aminoacidopathies
autosomal recessive inherited errors of matabolism
an enzyme defect inhibits the ability to metabolize certain amino acids
cause severe medical complications
- including brain damage
- toxic amino acids or their by-products build up in the blood
newborn screening tests are available for some to provide early treatment & therapies
Phenylketonuria (PKU)
the most well know aminoacidopathy
absence of the enzyme phenylalanine hydroxylase (PAH)
- converts phenylalanine to tyrosine
chronically elevated levels of phenylalanine can cause permanent brain damage
patients have a characteristic” mousy” odor to their urine
testing is done before newborns leave the hospital
- fluorescence of a complex formed between phenylalanine, ninhydrin & copper on a dried blood filter disk is measured
- positive results must be verified by another method
treatment : low phenylalanine diet
Tyrosinemia
3 types caused by deficiencies in enzymes in the metabolic pathway of tyrosine
Type 1
- most severe
- symptoms appear in first few months of life
- failure to thrive*, diarrhea, vomiting, jaundice
- can lead to liver/kidney failure & nervous system problems
Type 2
- symptoms: light sensitivity, eye pain, painful skin lesions on palms & soles of feet
- half have impaired mental development
Type 3
- only a few cases worldwide
- treatment: protein-restricted diet, medications, liver-transplant
Alkaptonuria
deficiency of homogentisate oxidase ( HGD)
-needed for metabolism of tyrosine & phenylalanine
elevated levels of homogentisic acid ( HGA) in blood & urine
symptoms present during 3rd decade of life
- blue-black pigment of ears, nose, tendons (ochronosis) casued by a build up of homogentisic
- arthritis -like degeneration of joints
- urine that turns black when mixed with air due to oxidation of HGA
Ferric Chloride test detects HGA in urine
treatment : high does Vitamin C to sloe accumulation of HGA in cartilage
Maple Syrup Urine Disease ( MSUD)
Absence or reduced activity of branched -chain-alpha-ketoacid decarboxylase (BCKD) enzymes
- inhibits metabolism of 3 essential amino acids : leucine, isoleucine & valine
maple syrup odor to urine
symptoms in newborns: lethargy, vomiting, lack of appetite, failure to thrive
treatment: protein-restricted diet
Isovaleric Acidemia
deficiency of isovaleryl-CoA dehydrogenase (IVD) enzyme
-involved inmetabolism of leucine
asymptomatic patients have odor of sweaty feet
symtoms in newborns: lethary, vomiting, lack of appetite & failure to thrive
treatment : protein- restricted diet
Homocystinuria
deficiency of cystathionine ß-synthase enzyme
- needed in the metabolism of methionine
symptoms : near-sightedness, dislocation of the lens in the eye, osteoporosis, mental defects
treatment: protein -restricted diet & high doses of vitamin B6
Cystinuria
Mutation of gene responsible for the synthesis of a protein complex in the kidneys that reabsorbs cystine from urine
-elevated levels of cystine in urine
cystine forms stones in the kidneys, ureters or bladder ( reoccur throughout patients life)
symptoms : hematuria, flank pain( back), UTIs
treatment : increase fluid intake to minimize stone formation
Amino acid analysis
plasma can be collected in a heparin tube after 6-8 hr fast
urine & amniotic fluid can also be analyzed
**Thin-Layer Chromatography is the method of choice
Protein- Basic structure
proteins play a key role in every function of living cells: motion, biochemical reactions, cell structure, transport, antibodies
consists of C, O, H, N, S
Macromolecules :
contain 200-300 amino acids
range in molecular mass from 6000 Daltons to several million
four levels of protein structure
Proteins 4 levels of structure
primary
- the # & type of amino acids in a specific sequence
- correct sequence is required to function properly
secondary
- structures stabilized by hydrogen bonds between the amino acids within a protein
- ex. alpha- helix ( most), ß-pleated sheet
- adds strength & flexibility
tertiary
- overall shape or conformation
- 3-D resulting from interactions of side chains
- physical & chemical properties are related to its tertiary structure
quaternary
- structure that results from the interaction of multiple protein molecules or subunits held together by non-covalent forces
Proteins - Properties
Nitrogen content: 16% by weight in serum protein
protein can contain many ionizable groups ( can be positive & negatively charged) aka amphoteric
the acid or base groups of the side chains can exist in different charged forms depending on the pH of the surrounding environment
Generally as pH increases, deprotonation (transfer of H+) occurs:
-carboxyl groups (R-COOH) are converted to carboxylate anions
(R-COO^-)
- ammonium groups (R-NH3) are converted to amino groups ( R-
NH2)
Proteins -isoelectric point
isoelectric point (pI)
- the pH at which a protein has no net charge
- the # of positively charged groups = # of negatively charged groups
when the pH is greater than the PI, the protein has a net neg charge
when the pH is less than the PI, the protein has a net pos charge
most proteins have a PI in the pH range of 5.5-8
the solubility of a protein depends on: # & type of amino acids it has & the pH of the environment
generally the greater the charge the greater the solubility
Protein Synthesis
occurs in
- Liver for plasma protein
- plasma cells for immunoglobulins
proteins are being rebuilt constantly
- Hb every 120 days
- plasma protein every 10 days
in some tissue, hormones assist in controlling protein synthesis
Nitrogen balance
Amino acids cannot be stored
most proteins are repeatedly synthesized & then degraded, recycling the amino acids
a balance exists between protein synthesis ( anabolism) & protein breakdown ( catabolism )
nitrogen balance occurs through equal intake & excretion of amino acids
the excretion of excess nitrogen occurs through reversible reactions that are catalyzed by transaminases
- ammonia is produced which is converted to urea & excreted in the urine
- ketoacids that are produced are oxidized & converted into glucose or fat
Protein classification - functions( osmotic pressure, enzymes, hormones, immunoglobulins, structural)
FUNCTION : DESCRIPTION ( EXAMPLES)
Maintain osmotic pressure: maintain water distribution
throughout body( Albumin) Enzymes: catalyze biochemical reactions (Transaminases,
Dehydrogenases, Phosphotases )
Hormones: chemical messengers that control the actionof
specific cell & organs ( Testosterone, Growth hormone,
Cortisol)
Immunoglobulins: antibodies produced by B cells in bone
marrow. part of immune response ( IgG, IgM, IgA)
Structural ( Collagen, Elastin, Keratin )
protein classification - function ( transport )
Transport: Hemoglobin -->O2 Albumin --> bilirubin Transferrin --> iron ß- globulin --> lipids Ceruloplasmin --> copper
protein classification - functions (storage, energy source, coagulation, pH )
Storage: store metal ions/amino acids for later release (ferritin–
–>iron)
Energy source: supply energy to tissue ( Creatine)
Coagulation ( FIbrinogen )
Acid- Base Balance : act as buffers to maintain pH ( 7.35-7.45 )
protein structure - simple proteins
Simple proteins
contain peptide chains composed of only amino acids
can be globular or fibrous
Gobular
- globe-like, symmetrical proteins
- soluble in water
- function:transporters, enzymes, messengers
- examples: Albumin, hemoglobin & immunoglobulins
Fibrous
- long protein filaments, asymmetrical
- water insoluble
- Function: provides structure to cells
- ex. troponin & collagen
protein structure- conjugated protein
Conjugated Protein
consist of protein & non protein
the non amino part is called the prosthetic group
- can be lipid, carbohydrate, porphyrin, metal etc
Metalloproteins
- metal ion attached to the protein
- ferritin- contains iron
- ceruloplasmin - contains copper
Lipoproteins
- lipid attached to a protein
- HDL, LDL
GLycoproteins
- a carbohydrate attached to a protein
10-40% carbohydrate compostion = glycoproetin
ex. haptoglobin, alpha1-antitrypsin
> 40% carbohydrate compostion = mucprotein or proteiglycan
ex. mucin-lubricant
Nucleoproteins
- nucleic acids combined with proteins
ex. chromatin
